1. Technical Field
The present invention relates to a discharge lamp lighting device that controls a discharge lamp having two electrodes. In particular, the present invention relates to a discharge lamp lighting device that controls a discharge lamp used as a backlight for various display panels such as big screen television sets.
2. Related Art
Recently, a cold-cathode fluorescent lamp (designated as “CCFL” hereinafter) used as a backlight for a liquid crystal panel is prone to be long, since the liquid crystal panel is becoming larger in size. When a high voltage is applied across the CCFL through one electrode thereof to light up the CCFL, the CCFL may have non-uniform brightness along the longitudinal direction thereof.
Japanese Patent Application Publication 2004-241136 discloses a discharge lamp lighting device including a pair of inverter circuits, in which one of the inverter circuits as a master inverter circuit is connected to one of two electrodes of the lamp, and the other inverter circuit as a slave inverter circuit is connected to the other electrode of the lamp. The lamp is lighted up by applying a high voltage across the lamp through each of the electrodes. This method of lighting the lamp is designated as “a differential drive method”.
However, characteristics of the master inverter circuit and the slave inverter circuit do not always coincide with each other. Therefore, current flows supplied from the inverter circuits may become unbalanced even if the same voltage is applied across the respective inverter circuits to light up the CCFL by the differential drive method.
Accordingly, a method is suggested to adjust duties of output voltages supplied from the two inverter circuits, respectively, to equalize the amounts of current flows from the two inverter circuits. However, when this method is employed, the duties of the inverters are generally different from each other. Therefore, the inverter circuit which generates a larger duty pulse is required to have a larger derating, which raises a problem against downsizing of the discharge lamp lighting device.
To overcome the above-mentioned drawbacks, an object of the present invention is to provide a discharge lamp lighting device that can easily equalize amounts of current flows flowing into a discharge lamp through each of the electrodes of the lamp without enlarging a derating of the inverter circuit.